Vortex flow measuring devices are frequently used for measuring flow of fluids in pipelines, especially of gas flows or vapor flows in a high temperature range. In such vortex flow measuring devices, a flow impediment is arranged in the flow path in such a manner that the fluid can flow past on both sides of the flow impediment. In such case, vortices are shed on both sides of the flow impediment. Across a broad Reynolds number range, the vortices are in such case shed alternately on both sides of the flow impediment, so that a staggered arrangement of vortices arises. This staggered arrangement of vortices is referred to as a Kármán vortex street. Utilized in such case in vortex flow measuring devices is the principle that frequency of vortex shedding—the so-called vortex frequency—with which these vortices are formed is proportional across a broad Reynolds number range to the flow velocity of the respective fluid. Accordingly, from the registered frequency of vortex shedding, which is referred to in the following as the vortex frequency, and from a calibration factor characteristic for the particular type of vortex flow measuring device, the flow velocity can be determined.
In such case, a vortex flow measuring device includes, as a rule, a measuring tube, in whose flow path a bluff body is arranged as a flow impediment. In such case, the bluff body usually extends in a diametral direction either completely across or over a considerable part of the inner cross section of the measuring tube in such a manner that the fluid can flow past on both sides of the bluff body. In such case, as a rule, at least two shedding edges which support a shedding of vortices are formed on the two sides of the bluff body. During use, the measuring tube is inserted into a pipeline whose fluid flow should be measured, so that the fluid flow flows through the measuring tube and at least partially against the bluff body.
Additionally, the vortex flow measuring device includes, as a rule, at least one vortex sensor, which responds to pressure fluctuations produced by vortices. This is arranged downstream from the two shedding edges. The vortex sensor can, in such case, be arranged within the bluff body, or downstream from the bluff body, especially as a separate component. The pressure fluctuations registered by the vortex sensor are converted into an electrical measurement signal, whose frequency is directly proportional to the flow velocity of the fluid. If the density of the fluid is additionally ascertained, or if such is known, the mass flow of the fluid can be calculated from the flow velocity and the density.
Vortex flow measuring devices of the type described above are used above all for measuring single phase media, especially fluids (liquids, gases). In special applications, it can, however, also occur that two or more materials, especially two or more fluids of different densities and compositions are present in the flow measurement. This can, in such case, involve the same material (e.g. water), which is simultaneously present in two phases or aggregate states. In the following, for purposes of simplification, the first phase and the second phase of a two or more phase medium flowing in the pipeline are discussed, wherein the first and second phases represent the two main phases with the largest mass flow fractions. Other phases can be contained in one or also in both phases, especially as solid particles. In such case, the first and second phases of the flowing two or more phase medium can be different aggregate states of the same material—as is, for example, the situation in the case of water condensate in steam—or they can also be two different materials, such as, for example, sand entrained in a liquid, etc. The first and second phases are especially each a fluid (liquid, gas). The wall flow can, in such case, also be in turn formed of more than just one medium, especially two different materials. Reference is also in each case made to this variant in the case of the further developments explained below, even when this is not explicitly noted (through the statement of “at least a second phase”) each time. The invention is especially applicable in the case of such mixtures of two phases, in the case of which the two phases do not (or only slightly) mix and the density difference between the two phases is so high that, in the case of flow through a pipeline, the second phase at least partially flows as a wall flow, especially along a lower tube wall section in the case of pipeline extending in a horizontal or inclined manner. The invention especially relates to such combinations in the case of which the second phase is a liquid running on the bottom and the first phase is a gas.
It is known that, in vortex flow measuring devices, the occurrence of two or more media leads to measurement errors in the flow velocity ascertained from the vortex frequency.
An example of the occurrence of a medium with two different phases flowing in a pipeline is the formation of liquid rivulets in gas lines. In such case, a liquid which flows along a wall of the respective pipeline as a wall flow is referred to as liquid rivulet, or generally as a rivulet. This case is also especially relevant with regard to vapor lines (steam lines), in which a wall flow composed of water can form as a second phase. Besides liquid rivulets in gas lines, however, solid bodies capable of flowing in liquid flows—such as, for example, sand—can also be brought along, so that the sand flow (mixed with liquid) flows along a wall of the respective pipeline, similar to the case of the above mentioned liquid rivulets. If the pipeline flowed through is arranged horizontally or inclined (with respect to the direction of the force of gravity) and the wall flow (of the second phase) has a higher density than the first phase conveyed in the pipeline, the wall flow then flows as a rule along a lower section of the wall of the pipeline.
In such case, for many applications, it is desirable to detect, reliably and without considerably increased costs, the occurrence of a second phase in a flow of a first phase, and, in given cases, also to determine the proportion of the second phase, and especially its mass flow. This is especially the case in applications in which steam is transported over larger distances. The supplying of hot steam in pipelines is utilized in industrial plants especially for providing energy, wherein, for this purpose, a high steam quality, which corresponds to a low fraction of liquid water, is required. In such case, the requirement especially often exists that the steam quality lies above 95%. The steam quality is, in such case, given as the ratio of the mass flow of the steam fraction to the total mass flow composed of steam and condensed water. Hot steam conveyed in pipelines is also used in the field of oil transport.
There exist various fundamental ways by which the second phase of a flowable, two or more phase medium can be transported in a pipeline. As already explained, the second phase can flow as a wall flow—especially as a rivulet—along a wall of the relevant pipeline. Additionally, the second phase also can be carried in the flow distributed relatively uniformly in the first phase as droplets or particles. These two types of flow of the second phase can, depending on circumstance, also occur simultaneously, or each occur only individually. Besides these two states of flow, there are also other known states, such as, for example, slug or bubble flow.
As is subsequently explained in more detail, the present invention primarily concerns the problem of reliable and prompt detection of wall flow of a second, flowable phase that flows along a wall of a pipeline, especially detection of a rivulet.
In the publication US 2006/0217899 A1, a method is described for monitoring fluid flow in a pipeline by means of a flow measuring device arranged in the pipeline. In such case, signal properties—essentially the RMS value (Root-Mean-Square)—are analyzed across a broadband frequency range of the signal, in order generally to detect the occurrence of two-phase flow. In such case, the different types of flow, especially the two types of flow explained above, are not distinguished between. In the publication US 2006/0217899 A1, it is especially noted that, for determining the second phase, explicitly those fluctuations are taken into consideration which do not lie in the wanted signal frequency range, and conventionally are suppressed for a better registering of the wanted signal. In particular, in the publication US 2006/0217899 A1, it is determined that the signal amplitude, calculated as an RMS value of the signal spectrum across a broadband region with high-frequency signal fractions of the vortex flow measuring device decreases with an increasing flow of the second phase. Accordingly, in the context of calibration, it is proposed to correlate the broadband RMS-values ascertained from the spectrum, the registered vortex frequency and the flow rate of the second phase, in order to then be able during use to ascertain the flow rate of the second phase from the measured signal amplitude and the registered vortex frequency. A disadvantage of the method described in the publication US 2006/0217899 A1 lies in the fact that, in given cases, disturbances, as can typically occur in the form of vibrations, can strongly corrupt the broadband RMS-value, and the certain detection of a second phase is therewith not assured.